1. Field of the Invention
The present invention relates to an austenitic heat-resisting cast steel having superior low-cycle fatigue life and creep resistance at high temperature and to an exhaust manifold using the same.
2. Description of the Related Art
An exhaust manifold of an automobile is an exhaust pipe that collects and creates an outlet channel for exhaust gas discharged from different cylinders into a single flow channel. An exhaust manifold is subjected to very high thermal impact depending on the output power of an engine because it is located at a position when the exhaust gas discharged from a head is first received from the engine. Since an exhaust manifold does not have a cooling mechanism, such as cooling water like the engine does, the temperature of the exhaust manifold increases to about 800° C.˜900° C. upon engine acceleration, and rapidly lowers to the ambient air temperature when the engine is turned off. This procedure is repeated a few times per day and thus the thermal impact on the exhaust manifold is very severe due to the temperature irregularity which it is subjected to. Hence, the exhaust manifold must have very high durability.
The material that is currently available for the exhaust manifolds includes a high-temperature oxidation resistant cast iron material, for example, FCD-HS cast iron, SiMo cast iron and high-Ni austenitic cast steel containing a large amount of Ni. As such, the FCD-HS cast iron and the SiMo cast iron are being used by adding Si or Mo to a conventional spheroidal graphite cast iron material to improve high-temperature properties and oxidation resistance. The typical temperature range of an exhaust system using such heat-resisting cast iron is about 630˜760° C., in which the temperature of exhaust gas is about 700˜800° C. In this temperature range, the above materials have a fatigue life of about 200 cycles or less and a creep life of about 200 hours or less. In addition, although a high-Ni austenitic cast steel that has superior high-temperature properties compared to cast iron materials is being used, the commercial application thereof is limited by the price because expensive Ni is added in an amount of 10 wt % or more.
Furthermore, in accordance with increasing the output power requirements of automobiles and the strict controls of exhaust emission standards these days, the temperature of exhaust gas is continuously increased, and durability and quality are strictly controlled and thus the load which is applied to the exhaust system is increasing more and more each year. Hence, in the case of the exhaust manifold, a material with improved fatigue life and creep resistance in a higher temperature range is required. However, because of limitation of the addition of a large amount of Ni resulting in high manufacturing costs, there is a need to improve fatigue life and creep resistance necessary at high temperature while adding a minimum amount of Ni.
This related art is merely utilized to enhance understanding about the background of the present invention, and will not be regarded as conventional techniques known to those having ordinary knowledge in the art.